1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to an exhaust gas purification device for an internal combustion engine equipped with an exhaust gas purifying catalyst having an O2 storage capability.
2. Description of the Related Art
There has been known a technology for purifying three components, i.e., HC, CO and NOx contained in the exhaust gas by disposing an exhaust gas purifying catalyst such as a three-way catalyst having an O2 storage capability in an exhaust gas passage of an engine that is operated at nearly the stoichiometric air-fuel ratio. The O2 storage capability of the three-way catalyst stands for a function for absorbing and holding the oxygen component in the exhaust gas in the catalyst when the air-fuel ratio of the exhaust gas flowing into the catalyst is lean, and for releasing the absorbed oxygen when the air-fuel ratio of the exhaust gas flowing in is rich. As is well known, the three-way catalyst is capable of simultaneously purifying three components, i.e., HC, CO and NOx contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst lies within a narrow range near the stoichiometric air-fuel ratio, but is no longer capable of simultaneously purifying the above-mentioned three components when the air-fuel ratio of the exhaust gas is deviated from the above-mentioned range. When the O2 storage capability is added to the three-way catalyst, on the other hand, the three-way catalyst absorbs an excess of oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in becomes more lean than the stoichiometric air-fuel ratio, and releases oxygen when the air-fuel ratio of the exhaust gas becomes rich, making it possible to maintain the atmosphere of the three-way catalyst at near the stoichiometric air-fuel ratio even when the air-fuel ratio of the exhaust gas flowing into the catalyst is deviated from the stoichiometric air-fuel ratio. Upon purifying the exhaust gas of the engine operated at an air-fuel ratio close to the stoichiometric air-fuel ratio by using the three-way catalyst having the O2 storage capability, therefore, it becomes possible to favorably remove three components, i.e., HC, CO and NOx simultaneously.
There has further been known an NOx occluding and reducing catalyst which absorbs NOx (nitrogen oxides) in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean, and releases NOx which it has absorbed when the oxygen concentration in the exhaust gas flowing in becomes low.
An exhaust gas purification device using the NOx occluding and reducing catalyst has been disclosed in, for example, Japanese Patent No. 2600492. According to the exhaust gas purification device of this patent, the NOx occluding and reducing catalyst is disposed in the exhaust passage of the engine which operates at a lean air-fuel ratio so as to absorb NOx contained in the exhaust gas when the engine is operating at a lean air-fuel ratio, and to release NOx which it has absorbed when the amount of NOx absorbed by the NOx occluding and reducing catalyst has increased by executing a rich-spike operation in which the engine is operated at the stoichiometric air-fuel ratio or at a rich air-fuel ratio for a short period of time, in order to purify the released NOx by the reduction. That is, when the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the concentration of oxygen in the exhaust gas sharply decreases compared to that of the exhaust gas of a lean air-fuel ratio and the amounts of the unburned HC and CO components sharply increase in the exhaust gas. Therefore, when the engine operating air-fuel ratio is changed over to the stoichiometric air-fuel ratio or to the rich air-fuel ratio by the rich-spike operation, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst changes from the lean air-fuel ratio into the stoichiometric air-fuel ratio or the rich air-fuel ratio, and NOx are released from the NOx occluding and reducing catalyst due to a decrease in the oxygen concentration in the exhaust gas. As described above, further, the unburned HO and CO components are contained in relatively large amounts in the exhaust gas of the stoichiometric air-fuel ratio or the rich air-fuel ratio and, hence, NOx released from the NOx occluding and reducing catalyst are reduced upon reacting with the unburned HC and CO components in the exhaust gas.
Further, the exhaust gas purifying device disclosed in Japanese Patent No. 2600492 judges the amount of NOx occluded by the NOx occluding and reducing catalyst based on a value of an NOx counter which increases or decreases depending upon the operating conditions of the engine, and executes the rich-spike operation when the value of the NOx counter has reached a predetermined value, so that the NOx occluding and reducing catalyst will not be saturated with the NOx that are absorbed. The NOx counter of the above-mentioned patent estimates the occluded amount of NOx by increasing the value of the NOx counter at a predetermined interval by an amount of NOx absorbed by the catalyst in accordance with the operating conditions of the engine when the engine is operating at a lean air-fuel ratio, and by decreasing the value of the NOx counter at the predetermined interval by an amount of NOx released from the catalyst in accordance with the operating conditions of the engine when the engine is operating at a rich air-fuel ratio. That is, the amount of NOx emitted per a unit time from an engine that is operating at a lean air-fuel ratio is determined in accordance with the operating conditions of the engine (load on the engine, air-fuel ratio, flow rate of the exhaust gas, etc.), and the NOx occluding and reducing catalyst absorbs NOx of this amount per a unit time. Therefore, the amount of NOx occluded by the NOx occluding and reducing catalyst per unit time is proportional to the amount of NOx emitted from the engine per a unit time. According to the above-mentioned patent, the amount of NOx absorbed by the NOx occluding and reducing catalyst per a unit time during operating conditions of the engine is stored as an absorbed amount of NOx in advance, and the absorbed amount of NOx is calculated in accordance with the operating conditions of the engine at a predetermined interval when the engine is operating at a lean air-fuel ratio thereby to increase the value of the NOx counter. Similarly, furthermore, the amount of NOx released from the NOx occluding and reducing catalyst per a unit time when the engine is operating at a rich air-fuel ratio is determined in accordance with the operating conditions of the engine (air-fuel ratio, flow rate of the exhaust gas). According to the above-mentioned patent, therefore, the amount of NOx released from the NOx occluding and reducing catalyst per a unit time during the operating conditions of the engine is stored as a released amount of NOx, and the value of the NOx counter is decreased by the released amount of NOx at a predetermined interval when the engine is operated at a rich air-fuel ratio such as during the rich-spike operation.
According to the exhaust gas purification device disclosed in Japanese Patent No. 2600492, it is allowed to efficiently purify NOx when the engine is operating at a lean air-fuel ratio. However, a problem arises when a three-way catalyst having an O2 storage capability is added as a start catalyst to the device of the above-mentioned Patent No. 2600492.
A principal object of the start catalyst is to remove HC and CO components released in large amounts from the engine during starting. The start catalyst must be disposed in an exhaust gas passage at a position as close as possible to the engine so that its temperature rises and reaches the activated temperature within a short period of time after the start of the engine. When added to the exhaust gas purification device of the Japanese Patent No. 2600492, therefore, the start catalyst is disposed in the exhaust gasw passage upstream of the NOx occluding and reducing catalyst.
When the catalyst having the O2 storage capability is disposed as a start catalyst in the exhaust gas passage upstream of the NOx occluding an reducing catalyst, however, it has been found that NOx are released from the NOx occluding and reducing catalyst without being purified at the initial stage of the rich-spike operation when the rich-spike operation is executed in order to release NOx from the NOx occluding and reducing catalyst and to purify them by the reduction during the operation at a lean air-fuel ratio.
It is considered that this problem is caused by a delay in the change in the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst from a lean air-fuel ratio to a rich air fuel ratio at the time of executing the rich-spike operation, due to the O2 storage capability of the start catalyst.
That is, when the rich-spike operation is executed, the air-fuel ratio of the exhaust gas from the engine sharply changes from a lean air-fuel ratio to a rich air-fuel ratio. However, the start catalyst has the O2 storage capability. Therefore, when the exhaust gas having a rich air-fuel ratio flows into the start catalyst, absorbed oxygen is released from the start catalyst, and the air-fuel ratio of the exhaust gas flowing out of the start catalyst is maintained near the stoichiometric air-fuel ratio. Accordingly, despite the rich-spike operation being started, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst fails to acquire a sufficiently rich value until oxygen absorbed by the start catalyst is all released. Therefore, the air-fuel ratio is often maintained close to the stoichiometric air-fuel ratio at the beginning of the rich-spike operation.
As the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst changes from a lean air-fuel ratio over to a lean air-fuel ratio which is close to the stoichiometric air-fuel ratio, the oxygen concentration sharply decreases near the surface of the NOx occluding an reducing catalyst. As will be described later, the NOx occluding and reducing catalyst is holding NOx therein in the form of nitric acid ions bonded to an alkaline earth element (e.g., Ba) and the like. When the oxygen concentration decreases near the catalyst surface, however, NOx held near the surface of the NOx occluding and reducing catalyst are released from the surface of the catalyst rapidly. In this case, when the exhaust gas flowing into the NOx occluding and reducing catalyst are maintained at a lean air-fuel ratio close to the stoichiometric air-fuel ratio, NOx that are released are not all reduced but flow toward the downstream side of the NOx occluding and reducing catalyst since HC and CO are not contained in the exhaust gas in amounts sufficient for reducing all the NOx that are released. Due to the O2 storage capability of the start catalyst, therefore, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst reaches a rich air-fuel ratio in a delayed manner at the time when the rich-spike operation is executed, and unpurified NOx are released from the NOx occluding and reducing catalyst.
When oxygen absorbed by the start catalyst is all released as described above, the exhaust gas on the downstream side of the start catalyst acquires a rich air-fuel ratio the same as that on the upstream side of the start catalyst, and an exhaust gas having a sufficiently rich air-fuel ratio is supplied into the NOx occluding and reducing catalyst. Therefore, when a given period of time passes after the start of the rich-spike operation, NOx released from the NOx occluding and reducing catalyst are all purified on the catalyst, and no unpurified NOx flow out of the NOx occluding and reducing catalyst. If unpurified NOx flow out of the NOx occluding and reducing catalyst every time when the rich-spike operation is executed, however, there arises a problem in that the NOx purification efficiency of the system decreases as a whole.
Further, in an engine in which the engine operating air-fuel ratio is changed from a lean air-fuel ratio to the stoichiometric air-fuel ratio, or to a rich air-fuel ratio, depending upon the operating conditions of the engine, the air-fuel ratio of the exhaust gas from the engine can often be changed from the lean air-fuel ratio to the stoichiometric air-fuel ratio, or to a rich air-fuel ratio, without the rich-spike operation being performed. In this case, too, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst may be temporarily maintained at a lean air-fuel ratio near the stoichiometric air-fuel ratio at the time when the operation air-fuel ratio is changed due to the O2 storage capability of the exhaust gas purifying catalyst. Then, unpurified NOx are released in the same way as described above, and deteriorated exhaust gas are emitted.
According to the exhaust gas purification device disclosed in the Japanese Patent No. 2600492, furthermore, the amount of NOx occluded by the NOx occluding and reducing catalyst is estimated by using the NOx counter to judge the timing for releasing NOx from the NOx occluding and reducing catalyst. When the three-way catalyst having the O2 storage capability is added as a start catalyst to the device of the above-mentioned patent, however, another problem arises as described below in addition to the problem mentioned above.
That is, when the exhaust gas purifying catalyst having the O2 storage capability is disposed in the exhaust gas passage on the upstream side of the NOx occluding and reducing catalyst, it was found that the value of the NOx counter often does not correctly correspond to the amount of NOx occluded by the NOx occluding and reducing catalyst, in addition to the above-mentioned problem.
This problem is attributed to a delay in the change in the air-fuel ratio of the exhaust gas on the outlet side of the exhaust purifying catalyst due to the O2 storage capability when the air-fuel ratio of the exhaust gas flowing into the exhaust gas purifying catalyst has changed.
That is, in the device of the above-mentioned patent, the value of the NOx counter is increased when the engine is operated at a lean air-fuel ratio, and the value of the NOx counter is decreased when the engine air-fuel ratio is changed to the rich side. With the exhaust gas purifying catalyst having the O2 capability being disposed on the upstream side of the NOx occluding and reducing catalyst, however, the air-fuel ratio of the exhaust gas passing through the exhaust gas purifying catalyst does not change to a rich air-fuel ratio until oxygen stored in the exhaust gas purifying catalyst is all released due to the O2 storage capability even if the operating air-fuel ratio has been changed from the lean side to the rich side and despite the exhaust gas acquiring a rich air-fuel ratio. Therefore, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst on the downstream side of the exhaust gas purifying catalyst is maintained near the stoichiometric air-fuel ratio until all the oxygen is released from the exhaust gas purifying catalyst despite the operating air-fuel ratio being changed to a rich air-fuel ratio, and NOx are not released from the NOx occluding and reducing catalyst. Therefore, if the operation is executed to decrease the amount of NOx occluded by the NOx occluding and reducing catalyst from a moment when the operating air-fuel ratio is changed from the lean side to the rich side, as taught in the above-mentioned Japanese Patent No. 2400492, the value of the NOx counter becomes smaller than a true occluded amount of NOx. It is therefore so judged that the NOx are all released at a moment when the value of the NOx counter has decreased to a predetermined value (≈0) as a result of the rich-spike operation even if the true value of the occluded NOx does not decrease to the predetermined value. When the operation is resumed at a lean air-fuel ratio, therefore, absorption of NOx starts again from a state where NOx are still occluded by the NOx occluding and reducing catalyst. Further, when the value of the NOx counter is increased from this state, the NOx occluding and reducing catalyst will occlude NOx in amounts larger than a value of the NOx counter. When the start or end of the rich-spike operation is judged based on the value of the NOx counter, therefore, it may often happen that NOx are occluded by the NOx occluding and reducing catalyst by an amount greater than an expected amount, whereby the absorption efficiency of the NOx occluding and reducing catalyst decreases and the NOx occluding and reducing catalyst is saturated with NOx that are absorbed.
Moreover, a similar problem occurs when the engine operating air-fuel ratio changes from the rich side to the lean side. When the engine operating air-fuel ratio is shifted from the rich side to the lean side and the exhaust gas flowing into the exhaust gas purifying catalyst acquires a lean air-fuel ratio, the exhaust gas purifying catalyst absorbs oxygen in the exhaust gas due to the O2 storage capability. Therefore, excess oxygen in the exhaust gas is absorbed by the exhaust gas purifying catalyst, and the exhaust gas flowing into the NOx occluding and reducing catalyst on the downstream side of the exhaust gas purifying catalyst fails to acquire a lean air-fuel ratio while the exhaust gas purifying catalyst is absorbing oxygen; i.e., the NOx occluding and reducing catalyst absorbs no NOx. As the exhaust gas purifying catalyst absorbs oxygen up to its maximum oxygen storage amount and becomes no longer capable of absorbing oxygen in the exhaust gas, the exhaust gas flowing into the NOx occluding and reducing catalyst on the downstream side of the exhaust gas purifying catalyst acquires a lean air-fuel ratio, and the NOx occluding and reducing catalyst starts absorbing NOx. When the value of the NOx counter is increased while the exhaust gas purifying catalyst is absorbing oxygen, however, the amount of NOx really occluded by the NOx occluding and reducing catalyst becomes smaller than the value of the NOx counter, and the amount of NOx really occluded and the value of the NOx counter do not agree with each other.
In view of the problems in the related art as set forth above, the object of the present invention is to eliminate a delay in the change of the air-fuel ratio of the exhaust gas on the downstream side of the exhaust gas purifying catalyst from a lean air-fuel ratio into the stoichiometric air-fuel ratio or to a rich air-fuel ratio when the exhaust gas purifying catalyst having an O2 storage capability is disposed in an exhaust gas passage.
Another object of the present invention is to provide a means for correctly estimating the amount of NOx occluded by an NOx occluding and reducing catalyst in an exhaust gas purifying device in which the NOx occluding and reducing catalyst is disposed on the downstream side of the exhaust gas purifying catalyst having the O2 storage capability.
The objects as set forth above are achieved by an exhaust gas purification device for an internal combustion engine for changing, as required, the operating air-fuel ratio into the operation at a lean air-fuel ratio and to the operation at the stoichiometric air-fuel ratio or at a rich air-fuel ratio, comprising:
an exhaust gas purifying catalyst having an O2 storage capability disposed in an exhaust gas passage of the engine; and
a storage decreasing means for decreasing the amount of oxygen stored in said exhaust gas purifying catalyst by feeding the fuel that does not contribute to the combustion in the engine so that the air-fuel ratio of the exhaust gas flowing into said exhaust gas purifying catalyst is more enriched than the engine operating air-fuel ratio at the time when the engine is to be changed from the operation at a lean air-fuel ratio to the operation at the stoichiometric air-fuel ratio or at a rich air-fuel ratio.
According to this aspect of the invention, the fuel that does not contribute to the combustion is fed to the engine at the time when the engine operating air-fuel ratio is to be changed from a lean air-fuel ratio over to the stoichiometric air-fuel ratio or to a rich air-fuel ratio. Since the fuel does not contribute to the combustion, it does not burn and turns into an unburned HC component, and is emitted from the engine together with the exhaust gas. Therefore, the exhaust gas having an air-fuel ratio more rich than the engine operating air-fuel ratio and containing unburned HC in large amounts flows into the exhaust gas purifying catalyst. In this case, oxygen is released from the exhaust gas purifying catalyst due to the O2 storage function of the exhaust gas purifying catalyst. However, there is a limit on the rate of releasing oxygen from the O2 storage. If the exhaust gas that flows in contains unburned HC components in large amounts, the oxygen that is released is no longer sufficient for oxidizing all of the unburned HC components in the exhaust gas, and the air-fuel ratio of the exhaust gas on the downstream side of the exhaust gas purifying catalyst becomes more rich than the stoichiometric air-fuel ratio. That is, oxygen stored in the exhaust gas purifying catalyst is released and is readily consumed, whereby the air-fuel ratio of the exhaust gas on the downstream side of the exhaust gas purifying catalyst readily changes into a rich air-fuel ratio. This makes it possible to eliminate a delay in the change of the air-fuel ratio caused by the O2 storage capability of the exhaust gas purifying catalyst. The supply of the fuel that does not contribute to the combustion is terminated when the amount of oxygen stored in the exhaust gas purifying catalyst is decreased to a sufficient degree (i.e., when the amount of oxygen is decreased to such a degree that oxygen released from the exhaust gas purifying catalyst does not practically cause problems) explained before. In an engine having a direct cylinder fuel injection valves which directly inject fuel into the cylinders, the storage decreasing means may inject the fuel into the cylinders in the expansion stroke or in the exhaust stroke of each cylinder. However, in an engine having exhaust port fuel injection valves that inject the fuel into the exhaust port of each cylinder, the storage decreasing means may inject the fuel into the exhaust ports. The fuel that does not contribute to the combustion may be fed by the O2 storage decreasing means during the operation at a lean air-fuel ratio of just before the engine operating air-fuel ratio is changed or during the operation at the stoichiometric air-fuel ratio or at a rich air-fuel ratio immediately after the change.
According to another aspect of the present invention, there is provided an exhaust gas purification device for an internal combustion engine which executes the operation at a lean air-fuel ratio as required, comprising:
an exhaust gas purifying catalyst having an O2 storage capability disposed in an exhaust gas passage of the engine;
a NOx occluding and reducing catalyst disposed in said exhaust gas passage downstream of said exhaust gas purifying catalyst to absorb NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is a lean air-fuel ratio and to release the absorbed NOx when the air-fuel ratio of the exhaust gas flowing in becomes a rich air-fuel ratio;
a means for executing a rich-spike operation for changing the engine operating air-fuel ratio into a rich air-fuel ratio for a short period at the time when the absorbed NOx are to be released from said NOx occluding and reducing catalyst while the engine is operating at a lean air-fuel ratio; and
a storage decreasing means for decreasing the amount of oxygen stored in said exhaust gas purifying catalyst by further enriching the air-fuel ratio of the exhaust gas flowing into said exhaust gas purifying catalyst beyond the air-fuel ratio of that during said rich-spike operation for a predetermined period of time immediately after the start of said rich-spike operation.
According to this aspect of the invention, the air-fuel ratio of the exhaust gas flowing into the exhaust gas purifying catalyst is further enriched beyond the air-fuel ratio of that during the rich-spike operation for a predetermined period immediately after the start of the rich-spike operation when the rich-spike operation is executed for releasing NOx from the NOx occluding and reducing catalyst. Therefore, unburned HC and CO components are contained in the exhaust gas in amounts sufficient for consuming all oxygen released even during the period in which oxygen is being released from the exhaust gas purifying catalyst due to the O2 storage capability, and the air-fuel ratio of the exhaust gas becomes sufficiently rich on the downstream side of the exhaust gas purifying catalyst even during the period in which oxygen is being released from the exhaust gas purifying catalyst. Thus, the exhaust gas having a sufficiently rich air-fuel ratio are supplied into the NOx occluding and reducing catalyst on the downstream side of the exhaust gas purifying catalyst from the start of the rich-spike operation, and the unpurified NOx do not flow out of the NOx occluding and reducing catalyst. The air-fuel ratio of the exhaust gas flowing into the exhaust gas purifying catalyst immediately after the start of the rich-spike operation is so set as to contain the unburned HC and CO components in amounts sufficient for consuming all oxygen released from the exhaust gas purifying catalyst and for purifying all NOx released from the NOx occluding and reducing catalyst on the downstream side. The storage decreasing means may feed the fuel that does not contribute to the combustion to the engine, such as the one which injects the fuel into the cylinders during the expansion stroke or the exhaust stroke of the cylinder or the one which injects the fuel into the exhaust port, or may be the one which further enriches the engine operating air-fuel ratio beyond that during the subsequent rich-spike operation. The above-mentioned predetermined period of time is set to be a time sufficient for the absorbed oxygen to be all released from the exhaust gas purifying catalyst.
According to a further aspect of the invention, there is provided an exhaust gas purification device for an internal combustion engine which changes the operating air-fuel ratio from a lean air-fuel ratio to a stoichiometric air-fuel ratio or a rich air-fuel ratio as required, comprising:
an exhaust gas purifying catalyst having an O2 storage capability disposed in an exhaust gas passage of the engine;
an NOx occluding and reducing catalyst disposed in said exhaust gas passage downstream of said exhaust gas purifying catalyst to absorb NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and to release the absorbed NOx when the oxygen concentration in the exhaust gas flowing in has decreased;
an NOx occlusion amount-estimating means for estimating the amount of NOx occluded by said NOx occluding and reducing catalyst based on the operating conditions of the engine; and
an NOx occlusion amount correction means for correcting the occluded amount of NOx estimated by said NOx occlusion amount-estimating means based on the amount of oxygen stored in said exhaust gas purifying catalyst when said engine operating air-fuel ratio has changed.
According to this aspect of the invention, the NOx occlusion amount-estimating means estimates the amount of NOx occluded by the NOx occluding and reducing catalyst based on the operating conditions of the engine such as engine operating air-fuel ratio, flow rate of the exhaust gas, fuel injection, etc. The NOx occlusion amount correction means corrects the occluded amount of NOx that is estimated depending upon the amount of oxygen stored in the exhaust gas purifying catalyst. For example, when oxygen is stored in a large amount by the exhaust gas purifying catalyst, a change in the operating conditions of the engine (e.g., operating air-fuel ratio) appears as a change in the condition of the exhaust gas flowing into the NOx occluding and reducing catalyst in a delayed manner depending upon the amount of oxygen that is stored. Upon correcting the NOx occlusion amount depending upon the stored amount of oxygen (delay time from a change in the operating conditions of the engine), therefore, it becomes possible to correctly estimate the amount of NOx occluded by the NOx occluding and reducing catalyst based upon the operating conditions of the engine without affected by the O2 storage capability of the exhaust gas purifying catalyst.